Without limiting the scope of the invention, its background is described in connection with apodizing techniques for wavelength filters in wavelength division multiplexed (WDM) networks.
Wavelength Division Multiplexing offers tremendous opportunities in exploiting the enormous information capacity offered by optical fibers. Of paramount importance for WDM networks is the ability to efficiently route optical carriers of different wavelengths to various channels. In an optically transparent WDM network, this function is performed by a cross-connect of optical channels whose main element is an optical filter. When assessing the quality of a filter to be deployed in such networks, three parameters are of special importance, namely, the bandwidth of the filter, the flatness of the filter response, and the level of sidelobe suppression. Specifically, these parameters determine the minimum channel separation.
A single-stage filter with uniform coupling (the coupling coefficient is independent of the position along the propagation direction) exhibits a sinc-squared response with a first sidelobe level of approximately -9 dB regardless of the specific type of interaction, for example, acousto-optic or electro-optic. This filter response can substantially limit the minimum channel separation. In addition, the non-flat filter response in the main lobe causes the filter output to become extremely sensitive to deviations of the center wavelength from the design wavelength. These deviations occur due to unavoidable fluctuations in the device parameters or change in fabrication conditions.
Several techniques have been proposed to improve sidelobe suppression. D. Smith et al. (Electron. Lett. 1989! 25:398-399) propose cascading filters with uniform coupling. While permitting the suppression of sidelobes below approximately -19 dB, this approach produces severe sharpening of the main lobe, thus aggravating the problems associated with deviations in the center wavelength. Trutna et al. (Optics Letters 1993! 18:28-30) suggest employing nonuniform coupling in order to suppress sidelobes below -30 dB in an acousto-optic tunable filter. Bornholdt et al. (Appl. Phys. Lett. 1990! 57:2517-2518) proposed a meander type, in-plane directional coupler comprising curved waveguides. However, these designs are fundamentally limited with respect to the specific apodizing functions which can be implemented. Specifically, these designs can not realize alternating functions. The capability to realize alternating interaction, i.e., a predetermined spatial dependence of the coupling coefficient with negative sections where the coupling coefficient changes its phase by .pi., is of paramount importance to permit a synthesis of a step-like response with a flat top. Without such appropriately chosen negative sections, even though the sidelobes can be suppressed, the response top remains sharp. This has been demonstrated by A. Kar-Roy et al (IEEE Photonics Technology Letters 1992! 4:1132-1135) for the case of a filter with a focused acoustic wave, and by L. B. Aronson et al. (Optics Letters 1993! 18:1721-1723) for a structure with nonuniformly distributed birefringence.
J. Johnson et al. in U.S. Pat. No. 5,218,653 propose to taper the interaction-strength profile in an acousto-optical tunable filter with coupled waveguides for the surface acoustic wave. H. G. Song, (Applied Optics 1994! 33:7458-7460)presents a functional form of the spatial dependence required to obtain a step-like response with suppressed sidelobes and proposes the use of 180.degree. reversals in a structure with coupled acousto-optic waveguides. Baran et al. in U.S. Pat. No. 5,446,807 disclosed an experimental acousto-optic filter with a flattened spectral response. However, these approaches have two substantial drawbacks. First, in the case of coupled acousto-optic waveguides, it is extremely difficult to incorporate the necessary functional profile of weighted (apodized) acousto-optic interaction. It requires waveguides with weighted coupling, for example, with a spatially varied inter-waveguide gap, and/or with an artificially induced and appropriately adjusted damping coefficient, for example, using a cover plate made of a semiconductor material. The fabrication of such a structure becomes complicated. Second, acousto-optic devices are known to require substantially higher drive power than electro-optic devices. Finally, the length of interaction must be several coupling lengths for the coupled acoustic waveguides. This results in interaction lengths of the order of several centimeters at conventional acoustic-wave frequencies and moderate drive powers. Accordingly, the minimum device length is of the same order of magnitude.
Methods of implementing alternating interaction-strength profiles, especially with electro-optic tuning, are sought to enhance the response of devices intended to filter/switch light in various optical systems, including WDM networks. In the present disclosure and in the appended claims, the term "light" encompasses optical radiation in both the visible and invisible spectral range, for example, infrared.